It is known that the better silver catalysts for the vapor phase oxidation of ethylene to ethylene oxide are formed by depositing silver upon a suitable support or carrier. It is also known that the silver deposit in the best catalysts is in the form of extremely fine grains of silver. One method of obtaining such fine grains, or crystals, of silver is by employing a dispersing agent in the solution of the silver compound in which the catalyst support is to be impregnated.
To obtain such finely divided silver various methods have been suggested by the prior art. In U.S. Pat. No. 2,404,438 certain carboxylic acid salts of silver are taught as useful in preparing a finely divided particulate silver for a supported catalyst. Polymers of unsaturated carboxylic acids are employed in U.S. Pat. No. 3,758,418 for a similar purpose. More recently in U.S. Pat. No. 3,887,491 certain natural gums, e.g. karaya, are employed in the aqueous silver-containing solution.
Another group of compounds which act as dispersing agents has now been discovered which provides a catalyst which is much more selective while still maintaining, or even improving, the conversion obtained; a lower temperature is required to obtain the same results thus saving energy. One species of the generic compound employed is disclosed in U.S. Pat. No. 3,893,855. At Column 3, lines 1-15, structural formula "B" shows the 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane employed in the examples of the present invention. These compounds are used to complex silver in coatings employed as photosensitive elements. The preparation of such compounds has been described in a paper by N. F. Curtis, Journal of the Chemical Society, 2644, 1964.